The possible role of thymus involution in predisposing individuals to the consequences of immunological senescence has assumed great interest recently in view of the changing population demographics in our society. Although considerable information exists about the effects of decreased secretion of thymus humoral factors, relatively little is known about the roles that changes in the export of newly- formed T cells plays in immune senescence. Recently, results from our laboratory have shown that recent thymic emigrants (RTE's) and their immediate descendants in the rat can be selectively identified and isolated on the FACS by their expression of the Thy1.1 alloantigen, which is absent from mature T cells. In normal prepubertal rats, most RTE's had a CD4+8- or CD4-8+, TCR alpha beta hi RT6- CD45C- phenotype, and differentiated into Thy1- RT6+ CD45RC+ T cells within 7 days; whereas in antigen-stimulated rats most RTE's were partially activated (anergic?) lymphocytes, which expressed antigen-specific immunoregulatory (IR) activity, but did not proliferate to antigen or mitogen. We now propose to conduct detailed studies of the quantitative and qualitative changes in the output of RTE's in normal and antigen-stimulated rats as a function of progressive thymus involution. We also propose to determine the effect that the prevention or reversal of thymus involution has on the generation of RTE's, Specifically, we will attempt to determine if thymus involution is associated with alterations in: 1) the number of RTE's exported per unit time; 2)the phenotypic profiles of RTE's 3) the cell cycle kinetics of RTE's; 4) the functional properties of RTE's; 5) the ability of RTE's to phenotypically and functionally differentiate; and 6) the ability to generate immunoregulatory (IR) RTE's. To accomplish this, purified populations of RTE's and their immediate descendants will be isolated from normal rats before and at various stages during thymus involution and characterized phenotypically by 3- color FACS analysis. The shore-term fate (1-10 days) of RTE's will be studied by following their phenotypic differentiation proliferation apoptosis in a wave of RTE's released from the thymus after i.t. labeling with FITC. The functional capabilities of RTE's will be tested in in vitro assays for lymphocyte activation, lymphokine production, and helper suppressor cytotoxic activity. Limiting dilution analyses will be used to estimate the frequencies of reactive cells. In addition, the relative susceptibility of RTE's to the induction of tolerance with immobilized anti-CD3 will be assessed; and the immunoregulatory functions of IR-RTE's in antigen-stimulated (OA/ BGG in CFA) rats will be studied in a transwell culture system. Results obtained from animals undergoing thymus involution will then be compared with those obtained from age- and sex- matched rats in which thymus involution is inhibited (e.g. adrenalectomy and/or gonadectomy in prepubertal rats) or reversed (e.g. adrenalectomy, gonadectomy, or LHRH-agonist treated elderly rats). Similar studies will be conducted in adult mutant BUF/Mna strain rats, which display spontaneous thymus hyperplasia.